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Semantic Web Services 169
their services and consumers can find the available ones. The quality of service part of the
SOA stack includes the following elements. Policy is a set of rules under which the service
provider makes the service available to consumers. Security is the set of rules that might be
applied to the identification, authorization, and access control of service consumers invoking
services. Transaction is the set of attributes that might be applied to a group of services to
deliver a consistent result. Management is the set of attributes that might be applied to
managing the services provided or consumed.
Functions
Quality of Service
Business Process
Service registry
Service
Service Description
Service Communication
Protocol
Policy
Security
Transaction
Management
Transport
Figure 6-1. SOA stack (retrieved from [31]).
The main advantages of SOA can be summarized as follows:
– SOA provides loosely coupled services which can be thus very easily distributed. This
contributes to its effective usage in distributed, grid computing [18].
– SOA provides an effective way for software application integration.
– SOA uses platform independent building elements contributing to higher flexibility
of the software application.
Despite of that SOA offers several advantages there are cases when it is not suitable. First,
if the application is already built, its modification to SOA requires redesigning and reimplementation.
This means that there will be additional costs. The access to certain functionalities
has also disadvantages. If some modules are closely related and they communicate extensively,
the use of web services may be an unreasonable trade off. Web services are not
suitable also when the component is highly bound to a user interface. In each situation
concrete circumstances and requirements need to be analyzed.
6.1.2 Web Services
The term web service [3, 24] is nowadays a very often used word, although not always
in the same meaning. Even though there exist a lot of definitions of web services, there
exist discussions which attributes must the service have [41]. For us, the most appropriate
definition is the one, given by W3C: